1. Field of the Invention
The present invention relates to a charging circuit for efficiently charging a secondary battery with electricity of an output of a generator and a charger using this charging circuit. More specifically, it relates to a charging circuit etc. using a small-sized manpower generator.
2. Description of the Related Art
Recently, such a small-sized manpower generator has appeared which utilizes a motor to improve environment consciousness or accommodate shutoff of a battery of mobile appliances. It is commercially available as a charger combined with a radio or for use in charging of a cellular phone.
Such a charger is generally used for charging in a charging circuit shown in FIG. 1. In FIG. 1, a reference numeral 10 indicates a generator. In it, R indicates an internal resistor (output resistor) of the generator 10, r indicates a current limiting resistor, D indicates a backflow preventing diode, and B indicates a secondary battery. Further, Ke indicates a counter-electromotive voltage constant and ω indicates an angular velocity. Vo indicates a generated voltage and Vb indicates a voltage applied to the secondary battery B and the diode D.
In the case of a charger having a small-sized man power generator that utilizes a motor, its power generating capacity is determined mainly by specifications of a motor portion (generator 10). To increase the capacity, the electromotive voltage constant can be increased by increasing the number of turns of a coil wire. However, since in a case where a load is the secondary battery, an impedance at time when it is charged is small and cannot match an output impedance of the generator. As a result thereof, its power cannot be taken out effectively, so that it has been impossible to avoid a significant deterioration in efficiency only by increasing the number of turns.
If a charging current is large in a case where an output resistance of the generator 10 is large, this resistance component causes power proportional to a square of the current to be dissipated. On the other hand, power of electricity with which the secondary battery is charged is determined by a ratio between a terminal voltage and a charging current. The terminal voltage changes with the charging current but at a very small rate and so is roughly constant as shown in FIG. 2. This is because the impedance at time when the secondary battery is charged has a very small value of 1Ω or less.
FIG. 2 shows a relationship between a voltage of the secondary battery and a charging current when it is charged. As shown in FIG. 2, there is a linear relationship between the voltage of the secondary battery and the charging current. An equation of y=0.0004x+1.4051 is an approximate expression, which indicates the relationship between the voltage of the secondary battery and the charging current with which it is charged, and R2 indicates a degree of approximation. In this case, a resistance value of a nickel-hydrogen battery (Ni-MH battery) used as the secondary battery is 0.4Ω. In an actual circuit, to this resistance a resistance of the charging-current-limiting resistor and a resistance of the charging circuit are added, thus giving a total sum of about 1Ω. It is generally known that when a load resistance and an output resistance are equal to each other, impedances match each other, in which case the load resistance has a maximum dissipation power, with efficiency of 50%. Therefore, in a case where the output resistance of the generator 10 is large, a loss increases as a charging current increases, thus deteriorating the efficiency.
FIG. 3 is a graph for showing relationships between a load resistance and each of the generated power, taken-out power, and charging current. Here, it is supposed that the generated power voltage of the generator 10 is 14V and its output resistance is 90Ω. As shown in FIG. 3, to increase the current, the load resistance must be reduced to be small, in which case the power that can be taken out and the efficiency therefor are deteriorated greatly.
That is, to utilize generated power effectively, it is important to accomplish impedance matching, so that the number of turns of the coil wire is limited by a resistance component of the charging circuit including the battery. Conventionally, this problem has been coped with by first accomplishing impedance matching between the output resistor of the generator and the secondary battery and then increasing a revolution speed of the motor so that a larger amount of generated electricity may be generated. For example, to charge the nickel-hydrogen battery with electricity of a current of 0.5 A, the output voltage of the generator 10 is set to about 2.0V because a voltage at the battery terminal is 1.6V and Vf of the backflow preventing diode is 0.3V.
Therefore, according to this setting, the coil is wound so that the output resistance of the generator 10 may be 4Ω because of 2.0V/0.5 A, and then the revolution speed of the generator motor is so set higher as to permit the current of 0.5 A to flow therethrough. However, such the setting significantly suppresses a degree of freedom in design of the generator, so that it has been difficult to keep the revolution speed low for low-noise design while obtaining required power at the same time.
Further, in a commercially available generator, a charging current or a voltage of generated power has been detected and a light emitting diode (LED) has been used as a power generation monitor, thereby prompting a user to generate power in a set condition. This is because operating time of an appliance when it has been run at a prescriptive revolution speed of 120 rpm for one minute is defined as a power generating capacity of the appliance. Thus, the user has had to generate power while keeping in mind the prescriptive revolution speed and time in order to store the prescriptive power. This has burdened the user greatly.
From a viewpoint of environments it is important to utilize human energy effectively or, from a viewpoint of convenience of a mobile appliance, it is important to increase the operating time of the appliance for each unit of power generating time. That is, such a technology is necessary to acquire a required amount of generated electricity, at a smallest possible torque.
The amount of generated electricity can be obtained more by increasing the electromotive voltage constant of the generator, which means at the same time that the output resistance is increased. This leads to a need for a technology of efficiently taking out power to an outside even if the output resistance is large (e.g., in a case where a generator having a large internal resistance is utilized).
Further, a portable type manual charger has been proposed which has a constant voltage circuit for regulating a voltage generated by the generator to a constant value.
This portable type manual charger comprises a rotary manual handle, a generator for generating a voltage by rotating this handle, a constant voltage circuit for regulating a voltage generated by this generator to a constant value, an output terminal for charging a secondary battery with electricity of an output of this constant voltage circuit, and a detection circuit for detecting a predetermined value of voltage or current applied to the secondary battery to be charged, wherein the constant voltage circuit is constituted of a step-down type DC/DC converter.
Although this portable type manual charger has a simply-structure, and attains to low-cost one, a problem of a power loss due to the output resistance of the generator has not been solved. Further, it has been impossible to control a charging current based on an amount of generated electricity.